linux-kernel - Re: [PATCH -next] pwm: pca9685: Remove set but not used variable 'pwm'

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Date:   Mon, 3 Jun 2019 14:40:29 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Sven Van Asbroeck <thesven73@...il.com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        YueHaibing <yuehaibing@...wei.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-pwm@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] pwm: pca9685: Remove set but not used variable
 'pwm'

On Sun, Jun 02, 2019 at 10:18:15AM -0400, Sven Van Asbroeck wrote:
> On Sat, Jun 1, 2019 at 12:05 PM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> >
> > I didn't look into the driver to try to understand that, but the
> > definitely needs a comment to explain for the next person to think they
> > can do a cleanup here.
> 
> Certainly.

I agree.

> But if we do restore the old behaviour, there may still be problems.
> I'm unsure if the old synchronization was working correctly.
> See the example at the end of this email.

I think you are right. pca9685_pwm_request() should take the mutex as
long as it is requesting PWM.

> An intuitive way forward would be to use a simple bitfield in
> struct pca9685 to track if a specific pwm is in use by either
> pwm or gpio. Protected by a mutex.

A flag would probably be easier to understand than the magic we have
now. Or then wrap it inside function with an explanation comment:

static inline void pca9685_pwm_set_as_gpio(struct pwm_device *pwm)
{
	/*
	 * We use ->chip_data to convoy the fact that the PWM channel is
	 * being used as GPIO instead of PWM.
	 */
	pwm_set_chip_data(pwm, (void *)1)
}

static inline void pca9685_pwm_set_as_pwm(struct pwm_device *pwm)
{
	pwm_set_chip_data(pwm, NULL);
}